1. Field of the Invention
The present invention generally relates to a navigation system which supports driving of a movable body such as a car or the like by displaying the current position and the traveling direction of the movable body, and more particularly to a navigation system which includes a GPS (Global Positioning System) receiver, and which amends a position measured by the GPS receiver by using a map matching and calculates the current position with high accuracy.
2. Description of the Related Art
A navigation system installed for example in an vehicle and the like is required to measure the self vehicle position and the traveling direction, and then to exactly and quickly display information to the driver on a display unit, which information include a map information corresponding to the self vehicle position i.e. the current position, guide marks indicating the current position and the traveling direction, and the other information required by the user. As a position measuring device, there are two types of navigation systems. One is a GPS type navigation system which receives the electric waves from the satellites and calculates the current position. The other is a self-sustaining navigation system which calculates the current position by using the data from an angular velocity sensor such as a gyro, an azimuth sensor such as a geomagnetic sensor, and a mileage sensor for detecting a mileage. The GPS system is extremely useful, since setting the self vehicle position on the map is not needed and since an error in the position measuring is not accumulated, therefore high reliability is obtained.
The navigation system which displays the current position by using the GPS system includes map data including digitized road information, and carries out a map matching, which estimate the accurate position by using the data. The conventional map matching includes the steps of setting a predetermined deviation area, in which the measuring position would exist, around the measuring position, detecting roads by searching the road information in the deviation area, selecting the road which satisfies the requirements by calculating the distances from the measuring position to each detected road, and moving the current position on the map. The deviation area around the measuring position is experientially calculated and predetermined.
However, the detecting of the conventional map matching was carried out by searching the road information in the predetermined deviation area. Therefore when the measuring accuracy which changes according to various circumstances or driving condition was extremely low, the road which would have included the current position was located out of the predetermined deviation area and the accuracy in estimating the current position became low. On the other hand, when the measuring accuracy had increased, the road which would have not included the current position was located within the predetermined deviation area and was searched. Therefore, a burden for the calculation had increased.